1. Field of the Invention
The present invention relates to a fusing device used for an image forming apparatus such as copy machine, printer, facsimile, and composite machine thereof. The invention also relates to a fusing device that can heat transfer material in accordance with its paper width.
2. Description of the Related Art
On an electro-photographic image forming apparatus such as copy machine, printer, facsimile, and composite machine thereof, latent image corresponding to an original is formed on a photosensitive material, toner is applied to the latent image to transform it into a visible image, the visible toner image is transferred on a recording paper (transfer material), and then the toner image transferred on the recording paper is fixed.
A fusing device for fixing toner image in a manner like the above includes a heating roller type fusing device in which recording paper having transferred toner image is heated and pressed while it is held and conveyed between a heating roller containing halogen heater or the like and a pressing roller for pressing the heating roller. This type of fusing device is widely utilized because of its simple construction and also because of excellent fixability onto transfer material. Both heating roller and pressing roller are made of core metal coated with rubber layer.
A fusing device like the above involves a problem of longer warming-up time (WUT) since heat cannot be transmitted easily because of thick rubber layer and accordingly heating time of the heating roller becomes longer.
To solve the above problem, an electro-magnetic heating type belt fusing device is utilized. This belt fusing device is constructed as follows: an endless fixing belt is passed between a heating roller and a fixing roller; there is provided a pressing roller that presses the fixing roller via the fixing belt; by applying high-frequency AC current to an exiting coil installed near the heating roller, induction flux is so induced that the heating roller generates heat and consequently the fixing belt is heated; and toner image on a transfer material is fixed while the transfer material is held and conveyed in a nip formed between the fixing belt and the pressing roller.
Since the heating efficiency of electro-magnetic heating type is high and the heat capacity of fixing belt is small, warming-up time decreases. In addition, power consumption (energy consumption) decreases.
However, because the heat capacity of fixing belt is small, temperature drop due to radiation is remarkable and the heat conductivity is low. Accordingly, if a recording paper narrower than the width of the fixing belt is fed, temperature increases at both edges of the fixing belt where no paper passes through because heat is not taken by the recording paper. This temperature increase is accelerated in case of continuous paper feeding.
For example, if a narrow recording paper such as A4R size is continuously fed, temperature of the edges where no recording paper passes through increases. Then, when a recording paper of A4 size is fed, uneven gloss is caused on the toner image formed at the edges, wrinkle is caused on the paper, or toner at the edges offsets onto the fixing belt, which results in a problem that favorable fixed image cannot be obtained. In some cases, the fixing belt may deform due to temperature difference.
A known technique for solving the problem by cooling the edges of the fixing belt is to supply air from a cooling fan.
Since the above problem is caused also on an electro-magnetic heating type fusing device that does not employ fixing belt, there have been disclosed preventive measures such as the fusing device as set forth in the Japanese Application Patent Laid-open Publication No. 2003-215954, where a center coil is provided at the center inside the heating roller and additional edge coil is also provided at each edge inside and energizing time of the three coils is controlled in accordance with the width of recording paper to be fixed.
The method where the edges of the fixing belt are cooled by air from a cooling fan is not favorable because power is needed for cooling. Besides, a method employing cooling as well as heating is not acceptable at all in the present days where energy conservation is strongly needed.
In the fusing device as set forth in the Japanese Application Patent Laid-open Publication No. 2003-215954, control of the energizing time of the three coils is very much complicated but still heating evenly in the axial direction is difficult. Besides, use of three coils increases cost.